1. Field of the Invention
The present invention is in the fields of cables and cable production. More particularly, the present invention is related to a profiled insulation for cables and the method for making the same.
2. Description of the Related Art
Copper cables are used for a variety of tasks, such as power transmission and signal transmission. In such tasks, the choice of insulation is of particular concern. In the area of signal transmission, for example, twisted pairs of copper conductors used in data cables (e.g. LAN cables) must meet certain fire safety standards and be cost effective, while minimizing signal degradation. Such signal degradation may be caused by factors such as interference with adjacent conductors, and inductance with the insulation.
Thus, in developing copper wire signal cables, often having multiple twisted pairs of copper wire within the same jacket, there are the competing concerns of minimizing cost while maximizing signal strength and clarity.
In order for the cable to function properly, the impedance measurement between the two copper conductors of a twisted pair must be precisely maintained. This is achieved by insulating the conductor with a dielectric material. However, the dielectric material has a negative impact on the electrical signal and contributes to signal losses as well as other undesirable electrical phenomena. In addition, this dielectric material adds cost to the cable construction and often has a negative impact on cable fire performance in UL testing. Thus it is desirable to find ways to reduce the amount of dielectric material in proximity to the copper conductor without affecting the impedance between the two copper conductors forming the twisted pair.
Several approaches have been taken in the past to reduce the amount of dielectric material in proximity to the copper conductors without reducing the impedance of the twisted pair made from said copper conductors. For example, some manufacturers have replaced typical copper wire dielectric insulation with a foamed dielectric insulation which adds a gas component to the insulation. This yields a reduction in the amount of dielectric material necessary to maintain the impedance of the twisted pair. It is known that the typical gases used to foam dielectric materials have a dielectric constant close to 1 (most desirable), whereas all other dielectric materials known at present have a dielectric constant substantially greater than 1, so this approach would appear, at first glance, to aid in resolving the concerns. However, this method not only greatly increases the complexity of the extrusion process, but often requires additional manufacturing equipment. It is also much more difficult to manufacture a data communications cable with good electrical properties using this type of process.
Another method to reduce the amount of insulation while simultaneously maintaining the impedance between a twisted pair of conductors is to add openings (air or inert gas filled) within the insulation itself. However, prior art methods for producing such insulation with longitudinal air/gas openings have either completely failed due to extrusion designs that do not produce the intended results or have otherwise produced ineffective results due to inconsistencies in the stable production of the openings.
Yet another manner for maintaining the impedance between a twisted pair of conductors while reducing the amount of insulation material used within a signal cable is to use what is termed “profiled” insulation. Profiled insulation refers to an insulation that is provided around a copper wire conductor, the cross-section of which is other than substantially circular. Such examples of profiled insulation may include comb-tooth structures or other similar designs intended to both separate the conductors from one another while using less insulation than a solid insulator of similar diameter but yielding the same impedance between twisted pairs of conductors. However, even with this method there are a number of drawbacks. First, it is difficult to achieve the desired shapes of the contoured insulation. Many of the desired insulation shapes are either too difficult or impossible to make under typical copper wire insulation extrusion lines conditions. Moreover, even if a particular design can be made for the insulation, they are typically generated using a manner, such as a shaped extrusion die (FIG. 1), that provides an inconsistent product, caused by such factors as increased shear rate from the die, and other production line conditions that are caused by the equipment used to generate the profiled insulation.